1. Field of the Invention
The present invention relates to lubricating oil formulations having low pour points and good low temperature Brookfield viscosities.
2. Description of the Related Art
In the industry there is a current trend to lubricating oil formulations possessing and exhibiting enhanced low temperature performance, as evidenced by lower pour points and lower −40° C. Brookfield viscosities. This trend is especially relevant in transmission fluid formulations where good low temperature properties are essential to ensure adequate lubrication at low temperatures which may be encountered upon equipment start-up. Low −40° C. Brookfield viscosity is also essential for enhanced fuel economy.
US2007/0000807 (WO 2007/005094) teaches industrial lubricant and grease compositions containing high viscosity index polyalphaolefins (HVI-PAO). The use of HVI-PAO in such compositions provides improved shear stability, enhanced anti-wear performance, superior foam properties, energy efficiency, etc. HVI-PAO is prepared by the polymerization of alpha olefins using reduced metal oxide catalysts such as described in U.S. Pat. No. 4,827,064, U.S. Pat. No. 4,827,073, U.S. Pat. No. 4,990,771, U.S. Pat. No. 5,012,020 and U.S. Pat. No. 5,264,642. HVI-PAO can also be made using zeolite catalysts, activated metallocene catalysts or Zeigler-Natta (Z-N) catalysts.
HVI-PAOs are characterized by having a high viscosity index and one or more of a branch ratio of less than 0.19, a weight average molecular weight of between 300 and 45,000, a number average molecular weight of between 300 and 18,000, a molecular weight distribution of between 1 and 5, a pour point below −15° C., and a kinematic viscosity at 100° C. in the range of from 3 cSt to 15,000 cSt. Materials of the type are commercially available, such as Spectra Syn Ultra™ fluid from ExxonMobil Chemical Company. US2007/0000807 teaches that such fluids, preferably those with viscosity indexes of 130 or greater, are particularly useful in formulating gear oils, circulating oils, compressor oils, hydraulic fluids, refrigeration lubes, metalworking fluids and greases. The HVI-PAO is used as such or can be mixed with at least one other ingredient selected from conventional PAO, esters, polyethers, polyether esters, alkylaromatic fluids, polyalkylene glycols, Group I base stock, Group II or Group III hydroprocessed base stock or lubricant oils derived from hydroisomerized waxy stocks, such as slack wax or waxy Fischer-Tropsch hydrocarbons.
Group II or Group III stocks are high viscosity index and very high viscosity index base stocks, respectively. The Group III oils generally contain fewer unsaturates and less sulfur than the Group II oils. In '807 it is indicated that the Group III stocks are materials such as those disclosed in U.S. Pat. No. 5,885,438, U.S. Pat. No. 5,643,440 and U.S. Pat. No. 5,358,628. In '438 the stock is a high VI wax isomerate made by isomerizing slack wax, F-T wax, waxy feed such as petroleum, VGO or raffinate. In '440 the high VI material is made by a two-step process of hydrocracking-hydroisomerizing wax. In '628 the high VI material is made by the two-step process of hydrocracking-hydroisomerizing petroleum waxes.
It is stated that the lubricants of '807 have superior properties and performance features, including excellent viscometrics, high VI, low pour point, superior low temperature viscosities, thermal oxidative stability, etc. In the text, examples are given only of HVI-PAO per se or of mixtures of HVI-PAO in combination with conventional PAO and/or in combination with conventional mineral oils identified as a mixture of a 4 cSt paraffinic solvent dewaxed stock and a 3 cSt naphthenic base stock, both of which are Group I stocks. In examples of synthetic anti-wear gear oils in the 75W90 to 75W140 range, the formulations exhibited −40° C., Brookfield viscosities of from 61,500 cP to 147,600 cP and pour points of <−65° C. to −55° C. The formulations in these examples included base stocks which were a mixture of HVI-PAO, conventional PAO and other base stocks with higher polarity, especially Group V base stocks including esters or alkylated aromatics of lower viscosity (1.3 to 6 cSt).
US2008/0020954 is directed to a lubricant formulation comprising at least two base stocks; the first has a Kinematic Viscosity @ 100° C. greater than 300 cSt and a tight molecular weight distribution, while the second stock has a Kinematic Viscosity @ 100° C. of less than 100 cSt. The first stock is identified as a metallocene catalyzed PAO with a KV@100° C. of 300 cSt or greater while the second stock can be a low viscosity PAO, one or more Group V base stocks such as an ester, polyalkylene glycol or an alkylated aromatic while Group II and/or Group III hydroprocessed or hydrocracked base stocks or their synthetic counterparts such as PAO, GTL or similar base stocks or mixtures thereof are also embraced. It is stated that favorable improvements are observed when the first high viscosity stock is added to lubricating systems comprising primarily Group II, Group III and/or GTL base stocks (page 9, para. [0089]). Despite this statement there are no examples in which the second base stock is anything other than a PAO of lower kinematic viscosity in combination with alkylated naphthalene, phalate ester and adipate ester along with a gear oil additive package.
US2007/0142242 is directed to a method for improving the performance of lubricating oils of high viscosity comprising GTL base stocks in combination with an effective amount of a polyolefin fluid having a KV@100° C. in the range between about 10 to about 1,000 mm2/s. Such lubricating oils are described as exhibiting improved oxidation stability, low temperature properties, viscosity retention and shear stability/weight retention superior to that exhibited by GTL additized with conventional viscosity modifiers and superior to that exhibited by Group I and Group II oils additized with the same polyolefin fluid or with a conventional viscosity modifier. The polyolefin fluid can be made using a catalyst system comprising a metallocene and an aluminoxane. In the example, however, use is made only of either ethylene-butene copolymers of KV@100° C. of 28 mm2/s or 114 mm2/s or of PAO −100 mm2/s (neither of which are HVI-PAO) in combination with the GTL or Group II base stocks.
WO 2007/095392 is directed to a base oil having a low dynamic viscosity as measured by ASTM D5133 which comprises a blend of about 65 to 97.5 wt % of a paraffinic oil having a VI of 130 or greater, a KV@100° C. of about 3.8 mm2/s or greater and a pour point of −15° C. or less and an ester of lubricating viscosity in an amount of from about 35 to about 2.5 wt %. The text goes on to describe the paraffinic oil as being made from waxy feed stocks to meet the requirement of a Group III base stock. Waxy feed stocks are identified as being either conventional natural, mineral stocks or stocks produced by synthetic processes exemplified by gas-to-liquids synthetic process on the Kolbel-Engelhardt process or the Fischer-Tropsch process. Such waxy stocks are processed by hydrotreating, hydrodewaxing and hydrofining to produce the desired paraffinic oil stock. No mention is made of using HVI-PAO anywhere in the text or in any of the Examples.